Certain porous materials need to have liquid coatings applied. Difficulties arise if the coatings are to be applied only to part of the porous material, because transport of the liquid through the porous material will occur. The liquid will then be in regions where the presence of the liquid or its residue is undesirable. An example of this is a carbon-carbon composite brake disc, where liquid anti-oxidant material should be applied only to the non-friction surfaces and must not contaminate the friction surfaces.
Brake discs that operate at high temperatures, such as those used in commercial and military aircraft, should be manufactured from materials having high heat resistance and long wear characteristics. Such brake discs normally operate at temperatures that exceed 1300° F. and can reach 2000° F. Such brake discs are commonly made of carbon-carbon composite materials. However, carbon can oxidize at elevated temperatures, which can cause disc weakening and can lead to structural damage and/or reduction of brake disc life.
Anti-oxidants are usually applied to the carbon surfaces to protect carbon-carbon composite brake discs from oxidation, maintain disc strength, and avoid early disc failures. Anti-oxidants can affect the friction and wear characteristics of the disc, and thus extreme care is required to prevent the anti-oxidant coating from reaching the friction surfaces. Heavy anti-oxidant coating may be necessary for discs operating at temperatures exceeding 1000° F., which may require several repetitions of the coating procedure, thus increasing cost. The available methods to apply the anti-oxidant to the non-friction surfaces of a carbon-carbon composite disc include manual or robotic techniques with possible masking of friction surfaces, which can be slow, inefficient, and costly. In addition, these methods are ineffective because the carbon-carbon composite material may have an open pore structure that will promote transport of the liquid anti-oxidant materials to the friction surfaces.
For instance, U.S. Pat. No. 5,686,144 describes a process in which a friction face of a brake disc is masked by a plate to isolate and seal the exterior from liquids. The plate is a fluid-tight plate. In order to achieve fluid-tightness, the plate may have annular grooves near its inner and outer circumferences, with rubber O-rings located in the grooves. See FIG. 5 of the patent. Alternatively, the faces of the plates that are turned towards the discs may be provided with elastic beads or edges of molded rubber. See FIG. 6 of the patent. In another variation, the plates may be constituted by elastically deformable sheets, for example of rubber. See FIG. 7 of the patent. The patent teaches that it is also possible to seal the friction faces by forming a surface coating that can subsequently be peeled off. The masked disc is immersed in a bath of impregnating composition containing a substance that can form a protective layer against oxidation. Impregnation is effected by establishing a pressure difference between the pressure at the exterior of the exposed surfaces of the immersed brake disc and the pressure inside the internal open pore space of the brake disc. This forces the impregnating composition to penetrate into the internal open pore space of the disc to form an internal oxidation protection.